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Recent developments in polymer chemistry, particularly in free radical controlled polymerization, have allowed the preparation of bespoke stabilizers (of chosen functionality, chain length, morphology, etc.). For example, various stabilizers have recently been synthesized using Atom Transfer Radical Polymerization (ATRP) or Reversible Addition-Fragmentation chain Transfer (RAFT) polymerizations. In this section, several representative examples of such stabilizers are discussed; starting from simple polyethylene macromonomers to complex stabilizers designed via controlled free radical polymerization techniques.

 MMA dispersion polymerization in dodecane using polyethylene macromonomer as stabilizer

Kawaguchi et al. synthesised PMMA-g-polyethylene (PMMA-g-PE) and investigated their efficiency as steric stabilizer for MMA free radical dispersion in dodecane.[67] Using this macromonomer, they successfully produced submicron monodisperse PMMA latex particles.

 MMA dispersion polymerization in n-decane using PS-block-poly(ethylene-co-propylene) as stabilizer PS-block-poly(ethylene-co-propylene) (PS-b-(PE-co-PP)) was used for dispersion polymerizations of MMA- based monomers in n-alkane.[68,69] Both Hirzinger et al. [68] and Hölderle et al. [69] used commercial PS-b- (PE-co-PP) stabilizer that form micelles in n-decane. For this reason, the stabilization mechanism was expected to be different from that of a dispersion polymerization performed with a macromonomer as the stabilizer.

Hirzinger et al. investigated the micellization of the stabilizer in the oil phase in order to draw a correlation between colloidal dispersion properties and stabilizer micelle characteristics.[70] They also investigated MMA free radical dispersion polymerization in n-decane. [68,71,72] From their studies, they found that the PS block of Kraton 1701 acted as an anchor to link the stabilizer to the PMMA particles (the PS chains were entangled with the core-forming PMMA chains). Therefore, Hirzinger et al.[68] found that a colloidal particle grew from a micelle for a shell-to-core mass ratio > 0.25, while smaller amount of colloidal latex particles were produced at lower shell-to-core mass ratio. As found with macromonomers, an increase of stabilizer concentration led to smaller latex particles.

Figure 13. Chemical structure of 2-(5-methacryloyl-pentyl)-1,3-oxazoline monomer

Later, Hölderle et al. used the same stabilizer (Kraton 1701) at different concentrations to control the size of particles obtained from dispersion copolymerization of MMA and oxalinefunctional methacrylate (Figure 13) in n-heptane.[69]

These authors found that oxazoline monomer content and cross-linking density had a minor effect on the particle size, but affected the particle polydispersity. Increasing the oxazoline content led to narrower particle size distributions, while increasing the grafting density had an opposite effect. Oxazoline monomer incorporation also led to lower glass transition temperature (Tg). In comparison, stabilizer and monomer concentrations had a major

impact on the final particle produced. When the stabilizer concentration varied from 1.3 to 15 wt%, the particle size decreased from 520 to 110 nm, while a change in monomer concentration from 11.3 to 23.5 wt% led to an increase in particle size from 80 to 270 nm.

 MMA dispersion polymerization in CCl4/2,2,4-trimetylpentane using polyisobutylene as stabilizer

Williamson et al. used a polyisobutylene polymer as steric stabilizer for MMA free radical dispersion polymerization in 2,2,4-trimethylpentane and introduced various amounts of carbon tetrachloride (CCl4) in order

to study the effect of dispersing medium solvency.[73] The main observation was that particle size was dramatically increased by adding CCl4; from 1 m in pure 2,2,4-trimethylpentane to 8 m in a mixture of 2:1 (by

volume) CCl4:2,2,4-trimethylpentane (a maximum particle size of 13 m had even been observed). Another effect

of CCl4 addition was a decrease in polymerization rate. At high ratios of CCl4, competing polymerization

processes took place; classical growth of colloidal particles and solution polymerization due to high solvency of dispersing medium. The coexistence of these two processes led to lower polymer molecular weight and slower polymerization rate.

O O

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 MMA dispersion polymerization in iso-octane using polyisobutylene as stabilizer

Winnik et al. have been very interested in dispersion polymerization in non-polar solvent.[74,75,76] Their research was driven by industrial potential application that they identified as colored inks for electrostatic imaging system. However, not like all the other research groups, they have been interested in investigating the morphology of the final particle, i.e. how the stabilizer chains and the polymer of the core are organized and / or segregated on the particle surface as well as within the colloidal particles. To locate the interfaces between stabilizer chains and polymer chains Winnik et al. used fluorescence quenching between two fluorescent groups. The colloidal particles are made of PMMA and sterically stabilized by PIB chains in iso-octane. However, after purification (centrifugation / redispersion cycles) the particles were transferred in cyclohexane. For the fluorescence quenching experiments, particles were fluorescently labeled on the PMMA chains and on the stabilizer chains (PIB) by different dyes which allow fluorescent transfer. Fluorescent dyes usually used as comonomer were 1-naphtylmethyl methacrylate or 9-anthrylmethyl methacrylate.[74] The quenching is observed when fluorescent dyes are in very close proximity. Their experiment of fluorescence transfer allowed them to make several observations. First of all they showed by fluorescence decay measurements that labeled stabilizer where located within the core of the particles. They also showed that after swelling of the particles (due to temperature increase) more fluorescent decay could be observed. Because the swelling of the core was due to the presence of PIB chains, the core-shell model usually identified for such particles was incoherent. Winnik et al.

suggested a new model called “microphase” model (Figure 14).

Figure 14. “Core-shell” structure (left) vs. “microphase” model (right) (reproduced from reference [74])

 MA dispersion polymerization in isododecane using poly(2-ethylhexyl acrylate) as macroRAFT agent / stabilizer

Recently, Houillot et al. used controlled free radical polymerization by RAFT to design a macroRAFT agent which efficiently stabilised poly(methyl acrylate) (PMA) latex particles prepared via dispersion polymerization in iso-dodecane.[77,78] They designed poly(2-ethylhexyl acrylate) (P2EHA) stabilizers by bulk free radical polymerization controlled using dithiobenzoate (DTB) or trithiocarbonate (TTC) RAFT agents (Figure 15).

Figure 15 Different structure of RAFT agent used in the macro-RAFT agent design

Subsequently, they performed free radical dispersion polymerization of methyl acrylate (MA) in iso-dodecane using three different stabilizers; a non-reactive P2EHA, a P2EHA terminated with DTB (one reactive site) (P2EHA-DTB) and a P2EHA incorporating TTC (two reactive sites) (P2EHA-TTC). These authors demonstrated that presence of the RAFT agent on the stabilizer chain was crucial to allow for covalent grafting and thus efficient colloidal particle stabilization; the non-reactive P2EHA was unable to absorb onto the particle surface.[78] In addition, a kinetic study of the free radical polymerization in the presence of P2EHA-TTC and P2EHA-DTB showed that only the P2EHA-TTC stabilizer was able to control the polymerization leading to well- defined triblock copolymers forming small monodisperse core-shell colloidal particles (120 to 320 nm).[77,78] The use of P2EHA-DTB macro-RAFT agent as stabilizer at the lowest concentration did not provide control over the polymer structure (PDI =18) but led to the formation of small monodisperse particles (39 to 93 nm). However, the monomer conversion stayed low compared to the other macro-RAFT agent and an important retardation phenomenon was observed. When the P2EHA stabilizer concentration was increased, a better control of the polymerization was achieved but the colloidal particle polydispersity increased.

 MMA dispersion polymerization in hexane/dodecane using PMMA-poly(octadecyl acrylate) (block or random) as stabilizer

ATRP was used by Harris et al. to design a new stabilizer for MMA dispersion polymerizations in a mixture of hexane and dodecane.[79] Harris et al. prepared three random PMMA-co-poly(octadecyl acrylate) (PMMA-co- PODA) and three block copolymers (PMMA-b-PODA) of methyl methacrylate (MMA) and octadecyl acrylate (ODA) (at different monomer ratios) in toluene.[79] On the contrary to stabilizers prepared via RAFT polymerization, these polymers did not possess a reactive group and, as a result, could not be covalently linked to the particle surface. For this reason, block copolymers with long PODA blocks were expected to be more efficient stabilizers than the random copolymers.

The authors showed that all stabilizers were able to form stable colloidal particles in the range of 400 to 2730 nm but that drastic variations in particle characteristics (size, polydispersity) existed depending on the stabilizers properties (random or block, monomer ratio). However, no apparent correlations between the stabilizer

O S S O O S O S S TTC DTB S O O S n O S S O O O O S O O O n/2 n/2 P2EHA-TTC P2EHA-DTC

parameters and the colloidal particles properties were drawn. In addition, the fact that random copolymers were able to stabilize MMA dispersion polymerization in hexane/dodecane mixture were unexpected. This was probably due to the difference between the MMA and ODA compositions which led to PODA segments that were long enough to provide stabilization. This assumption was supported by the fact that the best random copolymer stabilizer was the one incorporating the highest ODA fraction.

4. DISPERSION POLYMERIZATION OF STYRENE-BASED MONOMERS IN NON-POLAR